Assigning unique ID to a subset of observations

I have a panel data with 1002 observations. N=334, T=3. My goal is to create 2 columns representing N and the year T. For example, the first 334 observations will have ID from 1 to 334 and a fixed value of T=1. The next set will have observations from 335 to 668 and T=2. I tried this piece of code but it did not work.